It sounds like the Windows installation has become corrupt, due to the restart not completely correctly (it was probably in the middle of updating some files on the hard drive when the power went off).
However, it's worth checking that the hard drive itself didn't die when the power went out. Switch on the PC, quickly press the F2 or Del key to go into the BIOS. Without knowing more about the PC itself, it's impossible to tell you were exactly this information will be, but somewhere in the BIOS will tell what hard drives are connected to the PC. If the drive is fried, it won't show up at all. If it is there, then it's a case of either reinstalling Windows from scratch or repairing it.
If you have the original Windows installation disk or USB drive, you can put that into the PC, go into the BIOS and tell it to boot from the disk/USB drive. From there you have the option to either repair the Windows installation or use the system restore to go back to an earlier setup that worked (but this assumes you were using that particular service).
If that doesn't work, then the only option is to reinstall Windows.